Three-step Mechanism Research Articles

Numerical investigation of turbulent swirling flames with validation in a gas turbine model combustor

Numerical investigation of turbulent swirling flames is performed in a model gas turbine combustor. The calculations are performed using the CFD code OpenFOAM. Large Eddy Simulation (LES) approach based on the Smagorinsky model is used as the main turbulence modelling strategy, whereas Unsteady Reynolds Averaged Numerical Simulations (URANS) are also applied, employing the Shear Stress Transport model as the turbulence model. Turbulence-chemistry interactions are modelled by the Eddy Dissipation Concept (EDC) and the Laminar Flamelet Model (LFM). In EDC, a three-step global reaction mechanism is used. In LFM, limitations of the standard non-premixed approach, based on the mixture fraction and the scalar dissipation rate, for lifted flames like the present one, is overcome by adding the progress variable as an additional dimension to the flamelet libraries. URANS is applied only with combination with LFM. LES is applied in combination with EDC and LFM. Special attention is paid to obtaining an adequate grid resolution. Predictions are compared with measurements. It is observed that LES provides a better accuracy compared to URANS, whereas the latter may still be seen useful, since its computational time is shorter. For LES, it is observed that EDC provides a similar, or even slightly better overall-accuracy compared to LFM. On the other hand, it is observed that LFM requires substantially shorter computational times compared to EDC. This makes LFM attractive especially for LES of real combustors requiring much larger grids and/or for cases where a detailed reaction mechanism is of interest.

Phase transition and hydrogen storage properties of Mg17Ba2 compound

Mg17Ba2 hydrogen storage alloy was obtained by induction melting, and its hydrogen storage properties and phase transitions have been investigated. The reversible hydrogen capacity of Mg17Ba2 compound was 4.0 wt% H2. A three-step dehydrogenation mechanism was revealed for the first time, and a new hydride (Ba2Mg7H18) was discovered during the dehydrogenation process. Enthalpy and entropy changes for the dehydrogenation of Mg17Ba2 were also calculated.

A density functional theory study of novel catalysts for the “green” synthesis of aziridines

Density functional theory calculations were used to elucidate the mechanism of aziridination involving ethylene and model catalytic nitrenoids, ClMNHCl (MTiZn). Using the BP86/CEP-31G(d) level of theory, the Gibbs free energy changes for each reaction step were calculated, and transition states were found for the proposed mechanisms. It was concluded that the metal plays a key role in determining the reaction pathway. The “late” 3d metals Zn and Cu undergo a two-step mechanism while the “middle” 3d metals (Ni, Co, Fe) undergo a three-step mechanism including an additional isomerization. Surprisingly, these middle 3d metals also exhibit two competitive pathways: the “imide” pathway and the “amide” pathway. Overall, this research suggests that the model catalysts are promising candidates for olefin aziridination under mild conditions and worthy of further investigation.

First principle investigations of organobismuth palladium-catalyzed C–C coupling reaction: mechanism, chemoselectivity and solvent effects

A first principle study of the cross-coupling reaction between 3-chloro-6-iodopyridazine and triphenylbismuth under palladium catalysis has been carried out. The approach selected for this investigation is the density functional theory within the framework of the PBE0 and B3LYP hybrid exchange–correlation functionals, with and without including empirical corrections for the dispersion forces at the GD3 level. For this reaction, a three-step mechanism including an oxidative addition, a transmetalation and a reductive elimination has been proposed. All intermediates and transition states have been fully characterized. The energy profile is deduced in gas phase and in solvent. Calculations suggest that the oxidative addition is the rate-determining step for the entire cross-coupling reaction.

Silicon Diphosphide: A Si-Based Three-Dimensional Crystalline Framework as a High-Performance Li-Ion Battery Anode.

The development of an electrode material for rechargeable Li-ion batteries (LIBs) and the understanding of its reaction mechanism play key roles in enhancing the electrochemical characteristics of LIBs for use in various portable electronics and electric vehicles. Here, we report a three-dimensional (3D) crystalline-framework-structured silicon diphosphide (SiP2) and its interesting electrochemical behaviors for superior LIBs. During Li insertion in the SiP2, a three-step electrochemical reaction mechanism, sequentially comprised of a topotactic transition (0.55-2 V), an amorphization (0.25-2 V), and a conversion (0-2 V), was thoroughly analyzed. On the basis of the three-step electrochemical reaction mechanism, excellent electrochemical properties, such as high initial capacities, high initial Coulombic efficiencies, stable cycle behaviors, and fast-rate capabilities, were attained from the preparation of a nanostructured SiP2/C composite. This 3D crystalline-framework-structured SiP2 compound will be a promising alternative anode material in the realization and mass production of excellent, rechargeable LIBs.

ChemInform Abstract: Lignocellulosic Biomass Pyrolysis: A Review of Product Properties and Effects of Pyrolysis Parameters

AbstractReview: 266 refs.

Reaction Mechanism of Mycobacterium Tuberculosis Glutamine Synthetase Using Quantum Mechanics/Molecular Mechanics Calculations.

This paper is devoted to the understanding of the reaction mechanism of mycobacterium tuberculosis glutamine synthetase (mtGS) with atomic detail, using computational quantum mechanics/molecular mechanics (QM/MM) methods at the ONIOM M06-D3/6-311++G(2d,2p):ff99SB//B3LYP/6-31G(d):ff99SB level of theory. The complete reaction undergoes a three-step mechanism: the spontaneous transfer of phosphate from ATP to glutamate upon ammonium binding (ammonium quickly loses a proton to Asp54), the attack of ammonia on phosphorylated glutamate (yielding protonated glutamine), and the deprotonation of glutamine by the leaving phosphate. This exothermic reaction has an activation free energy of 21.5 kcal mol(-1) , which is consistent with that described for Escherichia coli glutamine synthetase (15-17 kcal mol(-1) ). The participating active site residues have been identified and their role and energy contributions clarified. This study provides an insightful atomic description of the biosynthetic reaction that takes place in this enzyme, opening doors for more accurate studies for developing new anti-tuberculosis therapies.

CO oxidation catalyzed by silicon carbide (SiC) monolayer: A theoretical study

Developing metal−free catalysts for CO oxidation has been a key scientific issue in solving the growing environmental problems caused by CO emission. In this work, the potential of the silicon carbide (SiC) monolayer as a metal−free catalyst for CO oxidation was systematically explored by means of density functional theory (DFT) computations. Our results revealed that CO oxidation reaction can easily proceed on SiC nanosheet, and a three-step mechanism was proposed: (1) the coadsorption of CO and O2 molecules, followed by (2) the formation of the first CO2 molecule, and (3) the recovery of catalyst by a second CO molecule. The last step is the rate−determining one of the whole catalytic reaction with the highest barrier of 0.65eV. Remarkably, larger curvature is found to have a negative effect on the catalytic performance of SiC nanosheet for CO oxidation. Therefore, our results suggested that flat SiC monolayer is a promising metal-free catalyst for CO oxidation.

Numerical computations of premixed propane flame in a swirl-stabilized burner: Effects of hydrogen enrichment, swirl number and equivalence ratio on flame characteristics

This paper investigates the effect of hydrogen addition and swirl intensity and equivalence ratio on characteristics of premixed C3H8–air flame, in a model burner. The numerical simulation is carried out using Reynolds Average Navier–Stokes (RANS) technique with Realizable k–ε as a turbulence closure model. The turbulence–chemistry interaction scheme is modeled using FR/EDM with three-step global reaction mechanism. The study is performed with two lean regimes of Φ = 0.3 and 0.5 and four volumetric fractions of hydrogen XH2 = 10%, 20%, 40% and 80% for two swirl numbers Sn = 0.6 and 1.05. First, validations of the computational models with the experimental data are performed, and a good agreement is found. Second, the effect of H2 addition to extended lean flammability limits is addressed. Third, the differential diffusion effects on the accuracy of the predictions are studied and therefore should always be accounted for. Fourth, the effect of H2 addition on the flow development and the flame characteristics is investigated. Results indicate that the H2 addition affects strongly the flame structure. Two types of enriched flame are found, Balloon-type and M-type flames. The effects of H2 addition on the reaction rates of C3H8 and H2, the flow species and CO emissions are also analyzed.

Renewable production of value-added jojobyl alcohols and biodiesel using a naturally-derived heterogeneous green catalyst

The present article addresses the generation of value-added jojobyl alcohols and biodiesel using the underutilized renewable natural resources. The butanolysis reaction of jojoba oil required for its transformation to jojobyl alcohols and fatty acid butyl esters was performed over mussel shells derived calcium oxide as a heterogeneous base catalyst. This study systematically investigated the influence of temperature, time, n-butanol concentration and catalyst loading on the butanolysis process. The results of the present study indicated that n-butanol concentration had a major impact on the reaction. The maximum jojoba oil conversion of 96.11% was registered after 1800min of transesterification reaction performed at 85°C using butanol-to-oil molar ratio of 10:1, 12wt.% catalyst amount and 350rpm stirring intensity. A mathematical model for its kinetics was developed based on a three-step mechanism equivalent to the Eley–Rideal one. It was concluded that the butanolysis reaction occurred between the surface chemisorbed butoxide ions and jojoba oil molecules in the liquid phase, whereas, the overall process was controlled by the surface reaction.

Thermal stability and spontaneous breakdown of free-standing metal nanowires

We present a model for vacancy-mediated spontaneous breakdown of free-standing monatomic nanowire based on exclusively random, thermally activated motion of atoms. The model suggests a new two-step vacancy-mediated mechanism for nanowire rupture compared to the more complex three-step hole-mediated mechanism driving the disintegration of nanowire on crystalline surface. It also demonstrates that a free-standing nanowire breaks down much more rapidly than a nanowire on a substrate, because it cannot experience the stabilizing effect of the nanowire/substrate interactions. The rupture mechanism includes single atomic vacancy generation, preceded by appearance of weakly bonded active atoms. The analysis of the simulation data indicates that the active atoms act as a precursor of vacancy formation. These two successive events in the temporal evolution of the nanowire morphology bring the free-standing nanowire into irreversible unstable state, leading to its total disintegration.The present study also manifests an unexpected substantial increase of the nanowire lifetime with diminishing the strength of the atomic interactions between the nanowire atoms. The simulation data reveal three energy regions where a large oscillatory variation of nanowire lifetime is realized. The first region of strong atomic interactions is characterized by tight nanowire rigidity and short lifetime. The next, second region in the consecutive step-down of the attractive interatomic force is characterized by generation of wave-shaped morphology of the atomic chain, enhanced flexibility and dramatic increase of nanowire lifetime. In the last, third region, further weakening of the interactions returns the nanowire again to unstable, short-lifetime state. The observed phenomenon is considered as a “stick-like” to “polymer-like” transition in the nanowire atomic structure as a result of interaction energy variation. The enhanced flexibility reduces the nanowire free energy since it favors and facilitates the rate of entropy propagation in the atomic chain structure. The observed phenomenon opens a way for a new type atomic scale control on the thermal stability of both free-standing nanowire and nanowire on crystalline substrate. The present study also extends the validity of the three-step breakdown mechanism of nanowire on crystalline substrate to the specific case of thermally activated free-standing nanowire rupture not affected by any external forces.

Comparative study of radiation-induced damage in magnesium aluminate spinel by means of IL, CL and RBS/C techniques

A comparative study of damage accumulation in magnesium aluminate spinel (MgAl2O4) has been conducted using ionoluminescence (IL), cathodoluminescence (CL) and Rutherford Backscattering Spectrometry/channeling (RBS/C) techniques. MgAl2O4 single crystal and polycrystalline samples were irradiated with 320 keV Ar+ ions at fluencies ranging from 1 × 1012 to 2 × 1016 cm−2 in order to create various levels of radiation damage. RBS/C measurements provided quantitative data about damage concentration in the samples. These values were then compared to the luminescence measurements. The results obtained by IL and RBS/C methods demonstrate a two-step character of damage buildup process. The CL data analysis points to the three-step damage accumulation mechanism involving the first defect transformation at fluencies of about 1013 cm−2 and second at about 1015 cm−2. The rate of changes resulting from the formation of nonluminescent recombination centers is clearly nonlinear and cannot be described in terms of continuous accumulation of point defects. Both, IL and CL techniques, appear as new, complementary tools bringing new possibilities in the damage accumulation studies in single- and polycrystalline materials.

Silica-based photonic crystals embedded in a chitosan-TEOS matrix: preparation, properties and proposed applications

Our research involves the development of a pH-sensitive chitosan-tetraethyl orthosilicate interpenetrating polymer network (Chi-TEOS IPN) suitable for the introduction of ordered silica nanoparticles. The pH-sensitive properties of the IPN provide an innovative framework for ordered colloidal silica (SiO2) films known as photonic crystals. The pH-sensitive properties of chitosan occur naturally due to its cationic polyelectrolyte structure; cross-linking with TEOS provides mechanical strength and incorporation of the silica photonic crystal provides a means of optical detection for a pH-driven expansion mechanism. Chitosan-tetraethyl orthosilicate-silica (Chi-TEOS-SiO2) composites are formed through a three-step mechanism in which the silica photonic crystal and Chi-TEOS IPN hydrogel are synthesized separately and then combined to form the Chi-TEOS-SiO2 composite. The silica photonic crystals, IPN membranes and final composite structures were analysed separately through optical, mechanical and swelling studies. Through the successful fabrication of a Chi-TEOS-SiO2 composite we propose a membrane suitable for applications in environmental, chemical and biological sensing of pH.

Density functional theory study of the reactivity and electronic structure of the transesterification of triacetin in biodiesel production via a sulfated zirconia heterogeneous catalysis

This DFT study examined the interaction of a sulfated zirconia (SZ) slab model system (heterogeneous catalyst) and triacetin (a precursor in biodiesel production) using explicit methanol solvent molecules. Full geometry optimizations of the systems were performed at the B3LYP level of theory. Gibbs free energies provide insight into the spontaneity of the reactions along a three-step reaction mechanism for the transesterification of triacetin. Charge decomposition analysis revealed electronic charge transfer between the metallic oxide and the organic moieties involved in the reaction mechanism. Fukui indices indicate the likely locations on the SZ surface where catalysis may occur. The quadratic synchronous transit scheme was used to locate transition structures for each step of the transesterification process. The results are in agreement with the strongly acidic catalytic character of zirconium observed experimentally in the production of biodiesel. © 2016 Wiley Periodicals, Inc.

A Mechanistic Study of Halogen Addition and Photoelimination from π-Conjugated Tellurophenes.

The ability to drive reactivity using visible light is of importance for many disciplines of chemistry and has significant implications for sustainable chemistry. Identifying photochemically active compounds and understanding photochemical mechanisms is important for the development of useful materials for synthesis and catalysis. Here we report a series of photoactive diphenyltellurophene compounds bearing electron-withdrawing and electron-donating substituents synthesized by alkyne coupling/ring closing or palladium-catalyzed ipso-arylation chemistry. The redox chemistry of these compounds was studied with respect to oxidative addition and photoelimination of bromine, which is of importance for energy storage reactions involving X2. The oxidative addition reaction mechanism was studied using density functional theory, the results of which support a three-step mechanism involving the formation of an initial η(1) association complex, a monobrominated intermediate, and finally the dibrominated product. All of the tellurophene derivatives undergo photoreduction using 430, 447, or 617 nm light depending on the absorption properties of the compound. Compounds bearing electron-withdrawing substituents have the highest photochemical quantum efficiencies in the presence of an alkene trap, with efficiencies of up to 42.4% for a pentafluorophenyl-functionalized tellurophene. The photoelimination reaction was studied in detail through bromine trapping experiments and laser flash photolysis, and a mechanism is proposed. The photoreaction, which occurs by release of bromine radicals, is competitive with intersystem crossing to the triplet state of the brominated species, as evidenced by the formation of singlet oxygen. These findings should be useful for the design of new photochemically active compounds supported by main-group elements.

Metastable Nitric Acid Trihydrate in Ice Clouds.

The composition of high‐altitude ice clouds is still a matter of intense discussion. The constituents in question are ice and nitric acid hydrates, but the exact phase composition of clouds and its formation mechanisms are still unknown. In this work, conclusive evidence for a long‐predicted phase, alpha‐nitric acid trihydrate (alpha‐NAT), is presented. This phase was characterized by a combination of X‐ray and neutron diffraction experiments, allowing a convincing structure solution. Furthermore, vibrational spectra (infrared and inelastic neutron scattering) were recorded and compared with theoretical calculations. A strong interaction between water ice and alpha‐NAT was found, which explains the experimental spectra and the phase‐transition kinetics. On the basis of these results, we propose a new three‐step mechanism for NAT formation in high‐altitude ice clouds.

A two-stage Ce/TiO2–Cu/CeO2 catalyst with separated catalytic functions for deep catalytic combustion of CH2Cl2

For the deep combustion of dichloromethane (DCM), a novel two-stage Ce/TiO2–Cu/CeO2 catalyst was designed and its catalytic performance was investigated in this study. It yielded a remarkable catalytic activity. The degradation efficiency reached up to 97.0% at 330°C with less undesired CO, Cl2 and byproducts (CxHyClz) in dry air. The DCM conversion and the yield of CO2 maintained during the long time test, even in the presence of water. Based on the comparison of catalytic performance and catalyst characterization of Ce/TiO2 and (Ce+Cu)/TiO2, a three-step degradation mechanism for DCM was proposed. The mechanism consisted of adsorption and rupture of the C–Cl bonds, deep oxidation of C–H from byproducts and total oxidation of CO to CO2, which afforded the theoretical basis for the design of the novel two-stage catalytic system. In the novel two-stage Ce/TiO2–Cu/CeO2 system, the rupture of C–Cl and the total oxidation of CO were physically isolated. This separated arrangement not only avoided the decrease of acid sites happened on (Ce+Cu)/TiO2 catalyst, but also avoided the chlorine poisoning of TiO2 due to the strong adsorption of Cl on the CuO. Moreover, CuO itself had high resistance to the toxicity of Cl.

An evaluation of entecavir for the treatment of chronic hepatitis B infection in adults

ABSTRACTEntecavir is a nucleoside analogue of 2ʹ-deoxyguanosine whose intracellular triphosphate form inhibits replication of the hepatitis B virus. Entecavir is recommended as a first-line monotherapy option for nucleos(t)ide-naïve patients with HBeAg-positive or -negative chronic hepatitis B infection. Entecavir has a three-step mechanism of action: It maintains viral suppression with a greater than 90% chance of undetectable hepatitis B virus DNA during continuous therapy, improves liver histology, and reduces the risk of liver failure or hepatocellular carcinoma development. The safety profile of long-term entecavir therapy is favorable; however, its optimal treatment duration is unknown. Entecavir monotherapy is not a rescue option for patients with lamivudine/adefovir resistance or baseline lamivudine-resistant mutants; rather, combination treatment is recommended for patients with lamivudine/adefovir resistance.

Chemical precipitation granular sludge (CPGS) formation for copper removal from wastewater

Solid–liquid separation and the settleability of sludge are always issues in copper wastewater treatment by chemical precipitation. To improve the settling performance of chemical precipitation sludge, controlled double-jet precipitation (CDJP) was used to treat wastewater by generating chemical precipitation granular sludge (CPGS). Several parameters such as the pH, copper oxide (CuO) seed dosage, stirring rate, adding rate and polyacrylamide dosage affecting the formation of the CPGS and wastewater treatment were investigated. Results show that the Cu(II) removal efficiency is over 99% and the settling velocity of the CPGS can reach up to 8.1 cm s−1 under optimized conditions, i.e., pH of 11, seed type of common CuO, seed dosage of 2.5 g L−1, stirring rate of 600 rpm, adding rate of 0.5 mL min−1 and PAM dosage of 20 g L−1. This settling velocity was about 18 times higher than that of chemical flocculent sludge. More importantly, the CPGS formation mechanism was explored. A three-step formation mechanism was proposed involving fresh precipitate growth on the seed surface, flocculation, and rearrangement with stirring.

Mechanism for the formation and growth of carbonaceous spheres from sucrose by hydrothermal carbonization

We report a new three-step mechanism for the formation and growth of carbonaceous spheres by hydrothermal carbonization of saccharides using sucrose as a precursor material.